1. Field of the Invention
The present invention relates generally to the field of pulsed oscillator apparatus and more particularly to phase-locked or PRF coherent, pulsed radio frequency oscillator apparatus as are commonly used in radar systems.
2. Discussion of the Prior Art
Radar systems, as is well known, have been extensively used in military and civilian applications since about World War II. Representative radar uses include the location and tracking of airborne aircraft and missles and of such natural phenomenae as tornados, hurricanes and storm fronts.
As is also well known, radar operates generally by the emission of electromagnetic waves, ordinarily produced by a radio frequency (r.f.) oscillator. These emitted waves are reflected towards the source by many types of objects, including those mentioned above. The reflected waves are received and compared with the transmitted waves with respect to wave phase and/or frequency, the differences being used to compute the location and velocity, if any, of the reflecting object relative to the radar transmitter. For ease in discrimination between transmitted and reflected waves, the r.f. oscillator of the radar transmitter is typically pulsed so as to provide a train of time separated pulses, each of which is, however, sufficiently long to include a number of the waves at the oscillator frequency.
Since information as to a detected object's location and velocity often depends upon minute differences between the transmitted and reflected waves or signals, it can readily be understood that the transmitted signal should be time-invariant in frequency and each pulse should be phase coherent. Accordingly, pulsed r.f. oscillators used in radar systems are commonly locked in phase to a precise, pulsed output of, for example, a crystal-type oscillator which may be a pulsed repetition frequency (PRF) generator.
In the past, such phase locking of the pulsed r.f. oscillators as are used in radar systems, has involved the use of relatively extensive, associated high frequency digital circuitry, which, for example, may cause the oscillator frequency to be an integral multiple of the PRF generator pulse frequency. As a result, such radar systems are not only comparatively costly, but also require relatively large amounts of power.
The need, however, exists in various important applications for small, relatively low cost and low power consuming radar apparatus. Typical of such applications is scoring apparatus for airborne, towed targets and/or pilotless drones used for gunnery practice. For applications such as these, the airborne scoring radar, which is configured for determining projectile miss distances within a pre-selected range, is subject to damage or destruction by target hits. Therefore, low cost of the scoring radar system is particularly desirable as is small size so as to lessen the probability of being hit by projectiles. Low power consumption of the radar system may also be an advantage for on-board scoring radar.
A need consequently exists in many instances for small, low cost radar systems and hence for small, low cost pulsed r.f. oscillator apparatus associated with such radars.
An object of the present invention is, therefore, to provide a pulsed r.f. oscillator, the output signal of which is coherent with respect to an associated PRF generator output signal, by reshaping the output pulse signal of the PRF generator and injecting the reshaped signal into the r.f. oscillator.
Another object of the present invention is to provide a PRF coherent, pulsed r.f. oscillator apparatus in which the output signal of a PRF generator is reshaped to provide an oscillator triggering signal that has substantial harmonic power near the free running frequency of the oscillator so that the reshaped signal controls the phase of the oscillator signal and also triggers the oscillator on and off.
Still another object of the present invention is to provide a PRF coherent, pulsed r.f. oscillator apparatus in which the pulsed output signal of a PRF generator is reshaped to provide an oscillator triggering signal which has a very fast rise time.
Yet another object of the present invention is to provide a PRF coherent, pulsed r.f. oscillator apparatus in which the pulsed output signal of a PRF generator is reshaped to provide an oscillator triggering signal which has a rise time which is no more than several times longer than the period of the pulsed oscillator signal.
Further objects, advantages and features of the present invention will become apparent to those skilled in the art from the following decription when taken in conjunction with the accompanying drawings.